For me, first time went on for days - started off feeling like trapped wind and slowly got more intense. I started panicking once they'd been going on for a while though and totally lost control, at which point I had an epidural.
Second time also started off feeling like trapped wind, in fact was having such a shit day with my grumpy toddler that I didn't even want to admit to myself, let alone my husband, that I was having contractions for about 5 hours (so obviously not that painful as it would've been written all over my face). They got more intense, and I can't really describe what they feel like later in labour as they felt like nothing else has ever felt like for me.
I second what people are saying about working with the pain and not panicking. One v simple breathing exercise made all the difference for me second time round - through each contraction visualising a rectangle, breathe in, visualise going along the top of the rectangle, breathe out, down the right hand side, breathe in, along the bottom, breathe out, up back to the start, repeat until contraction is over. And long low controlled moans with the breaths out.
As for waters breaking, first time mine were broken for me. Second time they broke about 5 mins before 2nd stage of labour kicked in. Contractions definitely felt more intense after.
Hope it goes well. Try to relax, and just remember it might hurt, but it will end!